Abstract: The invention consists of an open enclosure with pairs of spaced and parallel horizontal channels extending between end members front and rear and having a source of illumination therein. Cartridges are slideable from one end between adjacent tracks and these cartridges carry replaceable opaque plates having transparent indicia thereon. The plates slideably engage the cartridge from the upper side of the cartridge and between spacer bars. One piece gates close off all of the tracks so that a single lock such as a padlock for the front and rear gates can lock the entire assembly against tampering or unauthorized removal of the cartridges and plates. An alternative embodiment permits loading of the cartridges from the upper longitudinal side for substantially permanent signs with limited side access and the gates are then situated along the upper edge. If ony one side of the sign is to be used, then a blank plate or panel is inserted on the other side spanning the entire rear side of the sign.